This invention relates to an electronic ballast apparatus for the ignition and operation of a plurality of gas discharge lamps, and more particularly to an improved high frequency electronic ballast for multiple discharge lamps which regulates the output voltage supplied to the discharge lamps despite the absence or inoperation of one or more of the discharge lamps of a bank of parallel connected lamps. The invention further relates to a method of igniting and operating multiple discharge lamps with a regulated lamp output voltage, i.e. multiple lamp independent lamp operation (ILO).
One form of high frequency electronic ballast for the operation of multiple gas discharge lamps is described in the copending U.S. application Ser. No. 09/467,596 filed Dec. 20, 1999 in the name of Chang et al, issued on Jan. 30, 2001 as U. S. Pat. No. 6,181,079 B1, and which is shown in the accompanying FIG. 1 of the drawings. This electronic ballast circuit basically consists of two building blocks. The front end is a boost converter for power factor correction and universal input line voltage regulation. The main components are a transistor power switch Q1, an inductor L1, a diode D5 and the DC storage capacitor C1 along with an EMI filter and the diode bridge rectifier interposed between the AC supply voltage (e.g. 60 Hz) and the boost converter. The transistor switch Q1 is periodically switched on and off by a control circuit 7 as a function of the voltage across capacitor C1 and the current flowing through the transistor switch Q1 and a series connected sensing resistor 6.
The back end is a typical voltage-fed half-bridge inverter loaded with a group of parallel connected discharge lamps via a resonant tank circuit L2-C3. The main components are the power switches Q2 and Q3, resonant components including capacitor C3, inductor L2 and possibly the magnetizing inductance of the output transformer T1. The capacitors Clp in the secondary circuit of the transformer T1 are usually provided in order to ballast the lamp current and to protect against possible lamp rectification at the end of lamp life. The operation of the power switches Q2 and Q3 is controlled by a high voltage control IC 11 as a function of current flow in the transistor switch Q3 and of the voltage on capacitor C3.
In order to achieve multiple lamp independent operation (ILO) in a circuit such as that shown in FIG. 1, the output voltage (Vo) applied across the multiple parallel connected discharge lamps is usually kept constant at an rms value that exceeds the ignition voltage of the loaded gas discharge lamps. The level of the lamp ignition voltage is higher than the lamp operating voltage and presents the hazard of electric shock in the case where one or more of the multiple discharge lamps is (are) absent from a multiple lamp fixture.
For example, in the case of a fixture supporting multiple fluorescent TL lamps such as those with the manufacturers designation F32T8/TL735, the reliable ignition voltage is about 550 V (rms). In order to achieve independent lamp operation (ILO), the output (lamp) voltage is usually regulated to about 550 V in the normal steady state operation mode of the lamps even when less than all of the discharge lamps are operating, i.e. in a four-lamp fixture, even if one, two or three of the lamps are inoperative or are removed from the lamp fixture, the output voltage is still regulated at the ignition voltage value of 550 V (rms). In this case, the open circuit voltage across the lamp connector terminals will be the ignition voltage, 550 V (rms) which is required for the ignition of a newly inserted lamp or lamps. This presents the electric shock hazard mentioned above, especially during the removal of a discharge lamp or the insertion of a new lamp in the lamp fixture.
This problem is further exacerbated in Europe where the IEC 928 safety requirement, e.g. Section 12 concerning protection against electric shock, states that xe2x80x9cFor ballasts whose output terminals are to be connected to 250 V rated components, the voltage between any output terminals and between any output terminal and neutral or earth shall decrease within 5s after switching on or beginning of the starting process to a value less than 700 V (peak), under both normal and abnormal conditions . . .xe2x80x9d. This 700 V peak value translates to 495 V (rms) for sinusoidal waveforms. Therefore, the steady state output voltage exceeds the open circuit safety voltage. A ballast that operates with a 550 V (rms) lamp output voltage during steady state operation would clearly violate the European electric shock safety requirements of IEC 928.
Attention is also directed to the Japanese abstract 5-283183 by the Toshiba Corp. for a Discharge Lamp Lighting Device and Lighting System. This abstract describes a multiple lamp apparatus which detects if one lamp is removed from a bank of two parallel lamps by the use of a voltage detection circuit and a lamp filament detection circuit. This is but one of several known schemes for lamp insertion/removal detection based upon the detection of filament current. Most of these prior art circuits provide circuit protection when a lamp is removed by turning off the electronic ballast or putting the ballast into a standby mode. It appears that JP-A5-283,183 falls into this category of ballasts because of the use of the AND logic gate circuit 30. This circuit is not applicable for determining the number of inoperative lamps in a multiple lamp apparatus.
It is therefore an object of the invention to provide a high frequency electronic ballast for operation of multiple discharge lamps with a regulated lamp output voltage irrespective of the number of lamps actually in operation (ILO).
It is another object of the invention to provide a high frequency electronic ballast for operation of multiple discharge lamps which simultaneously provides independent lamp operation while satisfying the electric shock safety requirements that are desirable in this type of apparatus.
Another object of the invention is to provide an electronic ballast of the type mentioned which also regulates, e.g. makes constant, the lamp current in the case where the number of operating lamps is variable, thereby extending the useful lamp life and improving the ballast efficacy for partial load conditions.
A still further object of the invention is an electronic ballast of simple and inexpensive construction that nevertheless makes possible the objects and advantages mentioned above.
Another object of the invention is to provide an improved method of operating multiple gas discharge lamps which achieves the objects of the invention described above.
The above and other objects and advantages are achieved in accordance with the present invention by independently operating a plurality of discharge lamps in parallel by means of a high frequency electronic ballast that regulates the output lamp voltage even if one or more of the total number of lamps is inoperative or is removed from its connection terminals.
The regulation of lamp output voltage is achieved by monitoring and detecting the level of total lamp filament current flowing in the circuit, which then provides an indication of the actual number of discharge lamps that are in operation. A reference voltage is generated that is determined by the level of the detected total lamp filament current. By means of a feedback loop, the lamp output voltage is compared with the generated reference voltage and the frequency of the lamp output voltage is automatically adjusted so as to maintain a fixed (constant) output voltage level irrespective of the number of discharge lamps in operation at any given moment in time.
When a discharge lamp is inserted into a fixture that holds the multiple lamp configuration, there will be a rise or jump in the total filament current which is sensed. A short higher reference voltage is generated and the feedback loop responds to momentarily generate a higher lamp output voltage at a voltage level sufficient to promote ignition of the inserted discharge lamp. This higher output voltage is generated for a short time duration that will ensure lamp ignition, for example, for a time period much less than 5 seconds e.g., 100 ms. After lamp ignition, the apparatus then automatically readjusts the output lamp voltage back to the fixed (constant) voltage level suitable for steady state operation of the multiple parallel connected discharge lamps.
In the event a discharge lamp is removed from the lamp fixture, the electronic ballast maintains the generated reference voltage at the same level (unchanged) as before and the lamp output voltage is maintained at a constant voltage level. In one variation of this scheme the generated reference voltage is momentarily reduced to a lower voltage level which results in a faster output voltage regulation by the circuit during the lamp removal operation.
It is also desirable to maintain the discharge lamp current constant irrespective of the number of lamps actually operating in a multiple lamp fixture at any given moment in time. Therefore, in another preferred embodiment of the invention, a reference voltage generation scheme is provided to prevent overdrive of the remaining lamps after one or more lamps in a lamp fixture become inoperative or are removed and not replaced immediately. In this embodiment of the invention, the steady state lamp output voltage varies dependent upon the actual number of discharge lamps that are in operation in the multiple lamp fixture. Dependent upon the actual number of operating lamps, the operating frequency of the electronic ballast circuit is automatically adjusted so that the steady state lamp output voltage is of a value such that the current in each operating lamp is fixed at its optimum operational value irrespective of the number of actual lamps in operation. Thus, dependent upon the number of operating lamps, a different circuit operating frequency is adjusted so that the steady state lamp voltage is adjusted in a manner whereby each lamp current is almost the same in accordance with the adjusted circuit operating frequency. Thus, there is a distinct operating frequency for each combination of operating discharge lamps.
Accordingly, it is another object of the invention to provide a high frequency electronic ballast circuit which provides almost constant lamp current irrespective of the number of operational lamps in a multiple lamp apparatus.
The foregoing and other objects, features and advantages of the invention will become apparent with reference to the following detailed description thereof in connection with the accompanying drawings.